1. Field of the Invention
The invention relates generally to sugar production. The invention relates more particularly to processes for extracting sugar from sugar-bearing plant material.
2. Technical Background
Sugar is an important product, as it is used not only as a foodstuff but also as a raw material for the production of ethanol. Sugar is produced industrially by the extraction and purification of sugar from sugar-bearing plant materials. A variety of plants bear sugar. Currently, the industrially most important sugar-bearing plant is sugar cane. Sugar cane is a member of the Saccharum genus, which in turn is part of the grass family. It has a high agricultural yield and a high sugar content, and can be processed with relative ease. Sugar cane juice has as its chief sugars the disaccharide sucrose and its component monosaccharides fructose and glucose. Of course, other plant materials, such as sugar beets, are also used to produce sugar. Much effort has been expended in increasing sugar production, for example by increasing crop yields, increasing the sugar content of sugar-bearing plants, and increasing the extraction of sugar from the sugar-bearing plants.
In most commercial sugar operations, sugar is released from sugar-bearing plant material through a physical process. For example, in one typical process for extracting sucrose and other sugars from sugar cane, the sugar cane is chopped, and ground and pressed several times to extract the juice. Imbibition water (e.g., provided as water or dilute cane juice) can be added in one or more of these milling steps to rinse additional sugar from the milled sugar cane material. Liquids are collected, concentrated and treated to remove impurities, thereby forming a mixture of sugars having sucrose as its chief component. Pure sucrose can be isolated by crystallization.
The milling process generally yields sugar extraction in the range of 92-94%, with the rest of the sugar remaining entrapped in the fibers of the cane plant material. While extractions in the low-90 percent range are rather high, even higher extractions could relate directly to reduced waste and higher profitability. Accordingly, one focus of engineering efforts has been to attempt to squeeze the last few percent of the sugar from the milled sugar cane material. A variety of complex milling processes have been developed to more efficiently break up the sugar cane material, thereby allowing the juice to flow more freely therefrom. However, these processes can often require equipment that is expensive and difficult to maintain, and still do not achieve maximal extraction. The use of alkaline and highly alcoholic extractant solutions as well as steam have also been suggested. These processes can be severely limited by cost and purification concerns.
In so-called diffusion processes, shredded sugar cane material is repeatedly washed with hot water, often under pressure. Diffusion processes suffer from many of the same shortcomings as do milling processes: insufficiently high extraction, equipment intensivity, and cost and purification concerns.
Accordingly, there remains a need for new processes for the extraction of sugar from sugar-bearing plant materials with high extraction efficiency.